In This Issue:
- European Corn Borer Adults Seen in Ohio
- Armyworms on Corn
- Soybean Aphid Update
- ALS-Resistant Ragweed Control in Non-GMO Soybeans
- Field-Edge Weed Control Prevents Future Problems
- Got Residual?
- Crop Germination and Emergence in Dry Soils
- Staging Development in Early Season Corn
- Sentinel Plot Report – June 4
- Morningglory Control in Roundup Ready/Glyphosate Tolerant Corn
European Corn Borer Adults Seen in Ohio
Authors: Bruce Eisley, Ron Hammond
European corn borer moths are flying throughout Ohio. Even though much of the corn in Ohio may contain the Bt corn borer gene, fields without the gene should be checked for first brood borers. If fields are nearing the whorl stage, then signs of early larval activity may be found if one inspects a significant number of plants. Egg masses may be found on the underside of corn foliage if one searches long and hard.
If whorl injury (shot holes and window-pane feeding) is apparent, then about 20 plants should be inspected at 5 locations to determine the percentage of stand exhibiting whorl injury. During scouting, a number of whorls should be pulled and opened to determine presence or absence of ECB larvae. When larvae are found, the average number of larvae per plant may be estimated based on the proportion of stand exhibiting whorl injury and the proportion of injured plants actually having larvae present.
If the number of larvae found exceeds an average of one or more per plant and the larvae have not yet begun to burrow into the stalks, the infestation may warrant a rescue treatment. Treatment information can be found at:http://ohioline.osu.edu/b545/pdf/b545.pdf
Additional information about ECB can be found on the web at: http://ohioline.osu.edu/ent-fact/0015.html
Armyworms on Corn
Authors: Bruce Eisley, Ron Hammond
Growers should be watching their corn fields adjacent to wheat fields or planted into grass cover crops, especially rye, for the presence of armyworm injury. Severe infestation of armyworm can reduce stand when an infestation occurs in the pre-whorl stage and cause significant defoliation when corn is in the whorl stage. Total destruction of a field of no-till corn can occur if a severe infestation is allowed to develop without application of a rescue treatment.
Detection of foliar feeding injury by armyworm on 15 to 20% of a stand should be regarded as an indicator of a potential problem, and the field should be rechecked within a few days to determine whether the impact of defoliation is increasing and a rescue treatment may be warranted.
Rescue treatment in corn may be needed if stand infestation is greater than 50% and larvae are not mature. During the day, armyworm larvae will most likely be found seeking shelter in the ground cover or possibly in corn whorls.
A list of labeled insecticides for armyworm on both crops is available at http://ohioline.osu.edu/b545/pdf/b545.pdf.
Soybean Aphid Update
Authors: Bruce Eisley, Ron Hammond
As of today 6/4/2007, Ohio can be added to the list of northern states finding soybean aphids in soybean fields. We just found aphids in a field in Wayne County, and have received reports of aphids in other counties. Also, numerous states to our north, and in Ontario, have also been finding them the past week on young soybean plants. Growers, especially in the northwest around the Toledo area and perhaps in northeast OH, should be reminded to begin checking their soybean fields for possible early season outbreaks. Although not the rule, these early season buildups can catch growers off-guard. Watch future issues of C.O.R.N. for developments over the next month.
ALS-Resistant Ragweed Control in Non-GMO Soybeans
Authors: Mark Loux
Common and giant ragweed populations are likely to be resistant to ALS inhibitors in many non-GMO soybean fields. Resistant plants will not be controlled by postemergence ALS inhibitors, including Classic, FirstRate, Synchrony, Pursuit, and Raptor. Many non GMO producers apply postemergence treatments consisting of an ALS inhibitor plus a PPO inhibitor (Flexstar, Cobra, or Phoenix).
It’s important to recognize that, where ALS resistance is known or suspected, full rates of the Flexstar, Cobra, or Phoenix should be used in these combinations to ensure most effective control of resistant plants. The ALS inhibitor in the mixture can control other weeds in the field, and can help control ragweed that are not ALS–resistant, but will not provide any control of resistant plants. Adjuvant selection should also be geared to maximize the activity of the PPO inhibitor.
It is possible that fields with ALS-resistant giant ragweed may require a second postemergence application of a PPO inhibitor, to control late-emerging plants and plants that were not completely killed by the first application. Making two postemergence applications is a more effective strategy than delaying the initial application until giant ragweed are large, which can reduce PPO inhibitor effectiveness. Where two applications of PPO inhibitors are likely to be necessary, we suggest use of Flexstar in the first application, followed by Cobra or Phoenix in the second application.
Due to its persistence in soil and possibility (though slim) of carryover to corn, Flexstar can only be applied once per season, and early-season application helps minimize the risk of carryover. The short soil life of lactofen, the active ingredient in Cobra and Phoenix, ensures that carryover is not a possibility even with late-season applications.
Field-Edge Weed Control Prevents Future Problems
Authors: Mark Loux
This is just a reminder of the importance of prevention in weed management programs. Many biennial and perennial weed problems spread into crop fields from adjoining fencerows, roadsides, ditchbanks, and the edges of woods. Application of herbicide to weeds in these areas at this time of the year can provide effective control and prevent seed production, limiting their spread. Herbicides that have utility for noncrop areas include Crossbow, glyphosate, Cimarron, and mixtures of 2,4-D + dicamba, among others.
Similarly, paying extra attention to the weed control in the first 20 feet or so of crop fields that adjoin fencerows and similar areas can prevent small infestations from turning into major problems. “Extra attention” to the edges of crop fields might entail use of higher glyphosate rates, making a second sprayer pass around the field later in the season to ensure control of giant ragweed and perennials, removing individual plants by hand, and cutting plants off at the surface and treating the cut area with herbicide.
Got Residual?
Authors: Mark Loux
About this time of the season, the crop should start to become an essential component of the weed management program in a field. Healthy and uniform crop development, and the resulting shading of the soil surface, helps ensure that late-emerging weeds are suppressed and unable to effectively compete with the crop. For example, our research shows that when postemergence herbicides are applied to corn that is about 14 inches or taller, there should be little need for residual herbicide activity due to the corn’s ability to suppress later-emerging weeds. In contrast, we have often observed problems with poor control of weeds that emerge after early postemergence, non-residual treatments applied when corn is less than 10 inches tall. This can even occur where residual herbicides were applied at planting, because applying postemergence herbicides to very small corn fails to take advantage of the full utility of a two-application herbicide system.
As a result of unfavorable growing conditions over the past month, there can be quite a range in the size and growth stage of the crop within fields, from plants that have just emerged to plants that are relatively large. Weed size dictates that many fields should be treated with postemergence herbicides at this time, but the crop may not be well developed enough, or uniformly large enough, to do its part for weed control. One solution to this problem is to apply postemergence herbicides now, and then again in several weeks to control late-emerging weeds. This approach makes the most sense in Roundup Ready crops, since it can be accomplished relatively inexpensively and late glyphosate applications should not injure the crop. Aside from the major disadvantage of over-reliance on glyphosate, the other disadvantage of this approach in corn is that the second application can be difficult to apply when the crop is large.
The second approach to the problem of an uneven crop is to include herbicides with residual activity in postemergence treatments, with the goal of having the residual herbicides control weeds until the smallest crop plants are large enough to help out.
Herbicides with residual activity that can be added to postemergence glyphosate applications in soybeans include the following:
Scepter – residual control of broadleaf weeds, including black nightshade and giant ragweed (except ALS-resistant). Very little residual control of grasses.
Pursuit – residual control of grass and broadleaf weeds, but not as effective as Scepter on ragweeds, cocklebur, or morningglory. Extreme is a premix of glyphosate plus Pursuit.
FirstRate – residual control of many broadleaf weeds, but not black nightshade. No residual grass control. Use a rate of at least 0.3 oz/A.
Classic/Synchrony – potential for residual activity on broadleaf weeds, although the rate of chlorimuron is too low to ensure effective control of more tolerant species. No residual grass control.
Flexstar – potential for residual activity on pigweed, waterhemp, morningglories, common ragweed. No residual grass control.
In corn, the following herbicides have postemergence and residual activity, depending upon rate applied:
Atrazine – residual control of broadleaf weeds. Suppresses or controls annual grasses (except fall panicum), but primarily at higher rates. Any treatment or product containing atrazine must be applied when the corn is less than 12 inches tall.
Atrazine/acetamide premix products (Bicep II Magnum, Harness Xtra, Guardsman Max, etc) – residual control of grass and broadleaf weeds. Maximum corn size for postemergence application ranges from 5 to 11 inches, depending upon product.
Callisto – residual control of broadleaf weeds. No residual control of grasses, with the possible exception of large crabgrass.
Hornet – residual control of broadleaf weeds. No residual control of grasses.
Impact – residual control of some broadleaf weeds.
Resolve– residual control of annual grasses and a few broadleaf weeds, including lambsquarters, pigweeds, smartweeds, and common ragweed. Maximum corn size of 12 inches.
Lightning (Clearfield corn only) – residual control of annual grass and broadleaf weeds.
Crop Germination and Emergence in Dry Soils
Authors: Jim Beuerlein, Peter Thomison
Unusually dry soil conditions have resulted in delayed and uneven emergence in many Ohio corn and soybean fields. Moreover, weekend rainfall was highly variable across the state and soil moisture conditions remain inadequate for germination and emergence in some fields. What are the prospects for crop establishment in such fields? We've been receiving questions about the viability of soybean seeds whose germination was “suspended” due to drying soil.
Seeds that have swollen by taking up water, but cannot complete germination due to lack of adequate soil water, may be unable to resume germination, once water is available.
The critical moisture content for germination of soybean seeds is 55%; for corn, it is 35%. When seeds are planted, they undergo a number of drying/wetting events that occur in the soil resulting from rainfall events. Once soybean seed moisture content exceeds 55% germination, it is committed to germinate. Any drying back at this point will result in death of the seed/seedling. Seeds in which the seedcoat is broken and a root emerges (i.e. radicle protrusion) would require a moisture content in excess of 55%. Any drying back below 55% moisture would be fatal. If the seed is hydrated to 70%, but dried back to 60%, germination would proceed and would not harm the seed. Once cell division occurs in the radicle tip (this is after cell elongation that causes the radicle to initially grow), the seed is committed to germinate. New cells need at least that much water to function or they will die.
To determine if seeds whose germination was suspended due to drying soil are still capable of germination, you can dig them up and wrap them in a moistened paper towel in a warm place to see if they germinate. Keep towels moist and count the number of seeds that germinate after 1-2 days.
If soybean seeds swell only slightly and the seed coat is not broken, they may be able to germinate if they get more water within a few days. One reason we are probably not hearing as much about swollen corn seeds failing to germinate is that the critical moisture content for corn is less than soybean although this is offset somewhat by the larger size of corn seed. Nevertheless, if the moisture content of the corn seed exceeds 35%, it will irreversibly initiate germination. Drying back below 35% would cause the corn seed/seedling to die.
To determine if seeds whose germination was suspended due to drying soil are still capable of germination, you can dig them up and wrap them in a moistened paper towel in a warm place to see if they germinate. Keep towels moist and count the number of seeds that germinate after 1-2 days.
Reference:
Nafziger, E. 2007. Seeds and plants in dry soils. the Bulletin: Pest mgmt & crop developmt information for Illinois. Univ. of Illinois. [On-Line]. Available at http://www.ipm.uiuc.edu/bulletin/article.php?id=740. (URL verified 5/29/07).
Staging Development in Early Season Corn
Authors: Peter Thomison
When estimating yield losses in corn due to hail, frost, and other types of plant injury, it’s essential to establish the stage of plant growth at the time damage occurred. In recent years, it’s also become increasingly important to know corn stage of development in order to use postemergence herbicides effectively with minimum crop damage. This knowledge could be particularly important this year if the dry conditions we’re experiencing in much of the state continue resulting in uneven growth of corn.
Several systems are currently used to stage vegetative growth in corn.
The "leaf collar" system is probably the method most widely used by university and seed company agronomists in the Corn Belt. With this method, each leaf stage is defined according to the uppermost leaf whose leaf collar is visible. The first part of the collar that is visible is the back, which appears as a discolored line between the leaf blade and the leaf sheath. The oval shaped first leaf is a reference point for counting upward to the top visible leaf collar. This oval shaped leaf is counted as the number 1 leaf when staging. If a plant has 4 visible leaf collars, then it is defined as being at V4. Normally a plant at the V4 stage will have parts of the 5th and 6th leaves visible, but only four leaves with distinct collars. A field is defined as being at a given growth stage when at least 50% of the plants show collars.
Another widely used staging method is the "hail adjustor's horizontal leaf method" developed by the crop insurance industry. Rather than using the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40 to 50% exposed and whose tip points below the horizontal. Typically a given "horizontal leaf" growth stage based on the hail adjustor's method will be 1 to 2 leaf stages greater than the collar method. From growth stage V1 through about V5 there is typically one additional leaf (above that leaf with the last visible collar) whose leaf tip is pointing below the horizontal. Beyond growth stage V5, two or more additional leaves with 'droopy' leaf tips will be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be a 8-leaf plant according to the hail adjustor's horizontal leaf method). One problem with the horizontal leaf method is that it is often difficult to identify the uppermost horizontal leaves in fields that have recently experienced severe leaf damage. Hail adjustors get around this problem because they usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.
Corn leaf stage is a more reliable indicator of corn development than plant height. This is especially true in a cool, wet spring when corn is growing more slowly from a height standpoint. Differences in tillage and soil type often have a pronounced effect on plant height but relatively little effect on the stage of vegetative development. For example, within a field, corn may be taller in those areas characterized by darker soil (with higher organic matter) than in areas with lighter soil, especially the clay knolls, yet plants in both areas of the field may be at nearly the same stage when counting leaf collars.
At about V6 stage, or 8-leaf stage of the hail adjustor's method, increasing stalk and nodal growth combine to tear the smallest lower leaves from the plant. This results in degeneration and eventual loss of lower leaves. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process.There may also be occasions when the lower leaves are hard to identify prior to V6 stage. When extensive early season leaf damage has occurred, identification of the first rounded leaf and subsequent leaf collars may be difficult.
Dr. Bob Nielsen at Purdue has described a method for predicting leaf stage development using accumulated heat unit or growing degree day (GDD) information. Given an understanding of corn leaf stage development and heat unit calculation, a grower can predict what leaf stage of development a particular field is at given its planting date and temperatures since planting. It is useful to know when the crop emerged, but if you do not you can estimate that event also. Corn emergence typically requires 100 to 150 GDDs.
Dr. Nielsen proposes that corn leaf developmental rates may be characterized by two phases. From emergence to V10 (ten visible leaf collars), leaf emergence occurs approximately every 85 GDDs. From V10 to tasseling, leaf emergence occurs more rapidly at approximately one leaf every 50 GDDs. Previously, about 60-65 GDDs were associated with the appearance of new leaf collars during vegetative growth.
Example: A field was planted on April 28, but you do not know exactly when it emerged. Since planting, approximately 785 GDDs have accumulated. If you assume that the crop emerged in about 125 GDDs, then the estimated leaf stage for the crop would be between V7 and V8. We arrived at this estimate by first subtracting 125 from 785 to account for emergence, then dividing the result (660) by 85 to equal 7.8.
Dr. Nielsen warns that these predictions of leaf stage development are only estimates. One of the factors that most influences the accuracy of these estimates is the existence of other growth-limiting stresses and conditions (nutrient deficiencies, compaction, etc.). Despite these potential drawbacks, this heat unit method may be useful in timing when plants will reach an approximate stage of growth.
References:
Nielsen, R.L.. 2007. Determining Corn Leaf Stages. Corny News Articles, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/VStageMethods.html(URL verified 6/4/07)
Nielsen, R.L.. 2007. Use Thermal Time to Predict Leaf Stage Development in Corn
Corny News Articles, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/VStagePrediction.html(URL verified 6/4/07)
Sentinel Plot Report – June 4
Authors: Anne Dorrance
Eighteen of the twenty-three sentinel reports indicated drought stress last week. No reports of soybean diseases in any of the plots, the dry weather has not been favorable for development of diseases to date. Some areas in Ohio reportedly received heavy rains over the weekend, since it has been dry for so long it may take a while for these pathogens to get moving.
For soybean rust, there was one new find in Texas, but the area is very limited and does not pose a threat to southern soybean production regions. Louisiana also has not reported any finds, but the one kudzu location, the rust is doing quite well. Southern Florida finally received some rains this weekend – it will take at least 10 days for new infections to take hold – so we will be monitoring this situation the third week of June to see if rust becomes active again. The risk for Ohio for soybean rust is still very low at this time.
Morningglory Control in Roundup Ready/Glyphosate Tolerant Corn
Authors: Mark Loux
(The following article was written by Dr. Bill Johnson, Purdue University, and will appear in this week’s edition of the Purdue Pest & Crop Newsletter)
It is common knowledge that morningglories are tough to control with many postemergence herbicides, including glyphosate. Morningglory control in corn appears to have become more problematic over the last couple of years due to reduced use of soil residual herbicides and the fact that it is a weed that emerges relatively late in the spring after soil residual herbicides have dissipated. Atrazine provides effective control of morningglories as a pre or postemergence herbicide. As adoption of Roundup Ready/Glyphosate Tolerant corn increases, it appears that growers have reduced reliance on soil applied atrazine premix herbicides.
The best options for controlling morningglories postemergence in corn will depend on the following factors:
1) corn size/growth stage
2) morningglory size
3) whether or not glyphosate is needed as a tankmix partner.
If glyphosate is needed as a tankmix partner for other weeds, corn is 12 inches or less in height and morningglories are small (runners less than 4 inches), tank mix atrazine at 0.75 to 1.25 lb ai/A and increase glyphosate rate to 1.125 lb ae/A. This treatment will also provide some residual activity for later emerging flushes of morningglory. Another approach for this scenario would be to use dicamba, Distinct or Status with glyphosate. However, the use of atrazine will result in more residual control of the morningglories.
State Specialists: Anne Dorrance, Pierce Paul,and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Mark Loux and Jeff Stachler (Weed Science), Robert Mullen (Soil Fertility). Extension Educators: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Keith Diedrick (Wayne), Steve Prochaska (Crawford), Todd Mangen (Mercer), Mike Gastier (Huron), Alan Sundermeier (Wood), Gary Wilson (Hancock), Steve Bartels (Butler), Bruce Clevenger (Defiance), Mark Koenig (Sandusky), Harold Watters (Champaign), Greg LaBarge (Fulton), Steve Foster (Darke), Steve Ruhl (Morrow), Wesley Haun (Logan), Ed Lentz (Seneca) and Jim Lopshire (Paulding).